1. Field of the Invention
The present invention relates to passenger aircraft video distribution systems and more particularly concerns control of RF signal levels of such a system.
2. Description of Related Art
Each passenger of a passenger aircraft may be provided with an individually controllable electronics box unit to enable personal selection from among a group of different audio signals and a group of different video signals. The audio signals, and also the video signals, together with their own audio, are transmitted to each of the passenger seats from one or more central audio and/or video sources. The various entertainment signals are modulated upon individual ones of a plurality of RF carriers of different frequencies and transmitted to the individual passenger seats via a series of transmission stations which amplify the several signals, split the signals into different groups for transmission to different areas of the aircraft and tap signals off for use at the passenger seat. It is essential to ensure that optimum RF input levels of the video signal are provided to the video tuners at the individual passenger seat units. If the RF level at any seat unit is too low the signal is weak, and video may be poor, exhibiting "snow". At least partly because each video signal is modulated upon its own RF carrier, if the RF signal levels are too high, excessive inter-modulation products may be generated which would be visible to the passengers using the video tuners.
One possible approach to handling this problem of ensuring proper RF power levels throughout the system is to provide an automatic gain control function for each of the amplifiers, with parameters determined by system level requirements. However, this is not practical in the environment of a passenger aircraft because the aircraft configuration is frequently changed. Thus airlines often add or remove seats from one row or column, add additional channel capability by including additional sources of video channels or audio channels, or otherwise reconfigure the entertainment distribution system. With such reconfiguration, video signals encounter additional cable loss or additional gain, thereby changing power levels in the system. Changes in power level may be of such a magnitude as to be beyond the range of the automatic gain control unit. For example, if additional loss is added to the system by lengthening of cables, automatic gain control circuits of downstream amplifiers may not have enough range to compensate for lowered RF signal levels caused by the added cable loss. Therefore a fixed range automatic gain control amplifier would not be adequate.
Another situation in which fixed automatic gain control is inadequate is the occurrence of a failure. For example, if one amplifier in the system degrades in such a way that its output cannot be increased to the required level, or if some other element fails so as to greatly increase the power loss in the system, it may be necessary to compensate by increasing levels upstream of the failed element. The various electronic stations, which contain the gain control amplifiers, are ordinarily not easily accessible, nor is there often an available technician who is sufficiently knowledgeable for adjustment and readjustment of RF levels and gains throughout the system. Further it is also desirable to report the nature and location of any failure or degradation of operation to a central location for diagnostic purposes. Present systems provide for no such diagnosis.
Accordingly, it is an object of the present invention to provide a multiple signal distribution system that avoids or minimizes above-mentioned problems.